Recent descriptions of electronic architectures for time based pixel light sensors have been descriptions of digital implementations. In these implementations, instead of converting an analog photo detector voltage at the end of a fixed integration time to a digital value, time-based sensors measure a duration for the photo detector voltage to reach a reference voltage after a reset. This duration is directly related to a light intensity that is incident on the photo detector, and can therefore be used to determine a digital value representing the measured power of the light incident on the photodetector.
In conventional digital implementations, the time resolution and voltage resolution are constants, and they are referred herein to as single-resolution systems. Single-resolution sensors have a number of performance limitations. For example, when the photo detector voltage is sampled and compared to a fixed reference voltage at equally spaced time intervals; i.e., at Ts, 2Ts, 3Ts, in order to determine the duration for the photo detector voltage to reach a reference voltage after a reset, the transfer function of the time duration output and the light intensity input is 1/x, which is non-linear, and results in a non-uniform resolution of the measurement of the light intensity. The nonlinear 1/x transfer function will compress the image brightness, which is undesirable for most visual applications. The ability to resolve different power levels at high light intensity is also significantly limited by the single time resolution in these systems.
Thus, an improved technique for digitally measuring light intensity in a light sensing pixel is needed.
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